Process for preparing polyarylates

ABSTRACT

Described herein is a process for preparing polyarylates of improved color which comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen atom, at a temperature of from about 260° to about 350° C. Optionally, the process may be carried out in the presence of a magnesium, manganese, or zinc catalyst.

BACKGROUND OF THE INVENTION

This invention is directed to a process for preparing polyarylates of improved color and having a reduced viscosity of from 0.5 to greater than 1.0 dl/g, which process comprises reacting the diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent, based on the weight of the polyarylate produced, of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen atom, at a temperature of from about 260° to about 350° C. Optionally, the process may be carried out in the presence of a magnesium, manganese, or zinc catalyst.

Polyarylates are polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl)propane (also identified as Bisphenol-A), and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.

Many processes have been described in the literature for the preparation of polyarylates. One such process is the diacetate process. In the diacetate process, a dihydric phenol is converted to its diester derivative which is then reacted with an aromatic dicarboxylic acid to form the polyarylate. However, heretofore, the diacetate process has been carried out by several different methods as illustrated in the following U.S. patents:

In U.S. Pat. No. 2,595,343 issued May 6, 1952, an aromatic bisphenol is first reacted with acetic anhydride to form bisphenol diacetate which is then condensed with an aliphatic dicarboxylic acid to form a polyarylate by an acid interchange reaction. The condensation reaction is performed in the presence of an acid catalyst, such as para-toluenesulfonic acid, phosphoric acid or sulfuric acid. However, this acid exchange reaction does not yield an acceptable product when an aromatic dicarboxylic acid is substituted for the aliphatic acid.

U.S. Pat. No. 3,225,003 issued Dec. 21, 1965, describes the preparation of novel copolyesters by reacting hydroquinone diacetate with a mixture of hexahydroterephthalic acid and hexahydroisophthalic acid in the presence of a catalyst, such as sodium acetate, at a temperature of 230° C., and in an organic solvent such as 1, 1,2-trichloroethane. The reaction is exemplified as being carried out in the presence of anhydrous sodium acetate catalyst and eutectic mixtures of biphenyl and diphenyl oxide (diphenyl ether) at 230° C. in three stages for a total reaction time of about 28 hours. Thus, this process requires long reaction times and several steps.

U.S. Pat. No. 3,317,464, issued May 2, 1967, describes the preparation of linear aromatic polyesters by the polycondensation of diphenols, or their diacetates, with polynuclear aromatic dicarboxylic acids. The examples of this patent describe the preparation of the polyester using catalysts, such as mixtures of p-toluene sulphonic acid and antimony trioxide, or butyl orthotitanate, optionally in acetic acid. The reaction is carried out in a molten mixture of reagents and catalysts by heating these together under sub-atmospheric pressure. The patent states, but does not exemplify, that the reaction may be carried out in solution, in inert solvents, such as alpha-methyl-naphthalene, biphenyl or diphenyl oxide.

U.S. Pat. No. 3,329,653 issued July 4, 1967 describes the preparation of high molecular weight linear condensation polymers, such as aromatic polyesters, aliphatic polyamides, and polycarbamides. These polymers are formed at or below the melting point of the polymer by carrying out the reaction while the reacting materials are suspended in an inert non-solvent medium, with a swelling agent for the condensation polymer which also needs to be present in the reaction medium. Example 7 of this patent describes the preparation of poly[2,2-bis(4-hydroxyphenyl)propane isophthalate] by heating a mixture of 780.9 g of the diacetate of Bisphenol-A, 415.3 g of isophthalic acid, 900 g. of Apco Inkol No. 0, 25 g of sulfolane swelling agent and 2.5 g of sodium methoxide catalyst. The reaction is held at reflux for 40 hours. The patent describes, as particularly effective swelling agents, sulfolane, diphenyl ether, and quinoline. Thus, this process requires the use of large quantities of a non-solvent, swelling agents as well as long reaction times.

U.S. Pat. No. 3,824,213 issued July 16, 1974, describes the preparation of halogenated aromatic polyesters by reacting in solution, an aliphatic carboxylic ester of a halogenated bisphenol, such as tetrachlorobisphenol-A with an aromatic acid mixture of terephthalic and isophthalic acids at a temperature of 220°-350° C., in the presence of a catalytically effective amount of a cobalt, nickel, or manganese salt of an aliphatic carboxylic acid. This patent describes that the esterification reaction may be conducted with said catalysts in a suitable solvent, such as a hydrocarbon, halogenated aliphatic or aromatic hydrocarbon or the like (i.e. a solvent which is inert under the reaction conditions employed). The solvents which are specifically mentioned in the patent are: diphenyl ether, benzophenone, dichloroethane and dichlorobenzene. This patent exemplifies that several prior art catalysts, i.e., antimony oxide, zinc chloride, magnesium, magnesium oxide, magnesium acetate, calcium acetate, triphenyl phosphine, sulfuric acid, phosphoric acid, hydrochloric acid, borontrifluoride, and trifluoromethyl sulfonic acid all yield unacceptable polymers, in that the polymers have low I.V. values. The particular cobalt, nickel or manganese salts, as described in this patent, are necessary to yield polyesters having an inherent viscosity of at least about 0.2, which is considered an acceptable viscosity in that patent.

U.S. Pat. No. 3,948,856, issued Apr. 6, 1976, describes an acid interchange polymerization process for producing an aromatic polyester by reacting substantially stoichiometric amounts of an aromatic diester with a dicarboxylic acid at a temperature of 220°-350° C., in a solvent, and in the presence of a catalyst, which is a mixture of a transition metal salt of a strong inorganic acid and a transition metal salt of an aliphatic carboxylic acid. The aromatic diester and aromatic dicarboxylic acid are reacted in the presence of a solvent which must have a boiling point above about 220° C. The solvents specified are: diphenyl ether, halogenated diphenyl ether, diphenyl sulfone, benzophenone, polyphenyl ethers such as tri and tetraphenyl ether, terphenyl, tetraphenyl and halogenated biphenyl.

U.S. Pat. Nos. 3,684,766, issued Aug. 15, 1972, and 3,780,148, issued Dec. 18, 1973, describe a variation of the diacetate process. In the patented processes, a prepolymer is formed from, for example, a diacetate, such as Bisphenol-A diacetate, and an aromatic acid, in the presence of a catalyst. The prepolymer so formed is then comminuted into small particles. These particles are then contacted with a crystallizing agent to crystallize the polyester. The crystallized polyester is heated in the presence of an inert gas and under reduced pressure to increase the inherent viscosity of the polyester. However, the processes described in these patents require multi-steps including the step of crystallizing the prepolymer.

U.S. Pat. No. 4,075,173 issued Feb. 21, 1978, describes the preparation of copolyesters by reacting an aromatic dicarboxylic acid, a diacetate of Bisphenol-A, and an acetate of p-hydroxybenzoic acid. Various processes for producing polyarylates by the reaction of Bisphenol-A and terephthalic and isophthalic acids are reviewed in this patent. The following process for producing polyarylates, identified as route (1), is described in column 2, of the patent: ##STR1##

This process is the diacetate process as described herein, or the "Acetate process" as defined in the patent.

Column 2 of the patent states:

"The route (1) is not desirable because the undesirable coloration and deterioration of polymer are particularly remarkable as disclosed in the above-mentioned literature."

Further, column 3 of the patent states:

"On the other hand, the route (1), Acetate process, is economically advantageous because the materials used are cheap and the operation is simple. For example, diacetate of bisphenol-A, a monomer for Acetate process, is synthesized by merely reacting acetic anhydride and bisphenol-A. Consequently, it may be said that, if the fatal drawbacks of Acetate process, coloration and deterioration, are solved, Acetate process will become the most superior process."

Thus, the skilled workers in the field of polyarylate chemistry realize that the existing processes for producing polyarylates have one or more deficiencies, including a color problem, and that a need exists to develop a viable diacetate process for producing polyarylates wherein coloration is improved.

In U.S. Pat. No. 4,075,173, a copolyester was prepared by the diacetate process by a solid-state polymerization of low molecular weight polymers without using crystallizing agents. The patentees state that their Acetate process is possible only when specific monomers are combined to form the prepolymer. These monomers are the diacetates of bisphenol-A, terephthalic acid and/or isophthalic acid and an acetate of p-hydroxybenzoic acid. The prepolymer is then converted to the desired high molecular weight polymer by solid state polymerization.

Thus, the diacetate processes for producing polyarylates by the procedures of the aforediscussed U.S. patents, and as stated in U.S. Pat. No. 4,075,173, are generally unsuitable since they are either economically unattractive (they have poor color) and/or produce a commercially unmarketable product.

Therefore, a need exists for an economical and practical diacetate process for producing high molecular weight polyarylates having improved color.

In the diacetate process for producing polyarylates, problems exist which must be additionally solved in order to have a viable, economically attractive process. One problem is that the production of polyarylates by the diacetate process using existing catalysts, solvents, techniques, etc. produces polyarylates which are colored. Thus, these polyarylates have limited uses, since they cannot be used to produce molded products where good color is required.

Another problem when a diester derivative of a dihydric phenol is reacted with an aromatic dicarboxylic acid in the molten state is that sublimation of the diacid occurs. This disrupts the stoichiometry of the reaction and the polyarylate produced is not of acceptable molecular weight. To prevent sublimation of the diacid, several techniques have been developed. These include the use of large amounts of solvents together with a variety of catalysts and generally long reaction times in the polymerization process. However, these techniques are quite costly and do not provide an optimum process. Another problem when a diester derivative of a dihydric phenol is reacted with an aromatic dicarboxylic acid in the molten state is that the viscosity of the system increases dramatically towards the end of the reaction and therefore the reaction becomes diffusion controlled (the molecules are not close enough to insure rapid reaction) rather than kinetically controlled. Also, the polymer product is difficult to handle (i.e., removal from the reactor) due to this high viscosity.

Yet another problem in the production of polyarylates by the diacetate process is that a carboxylic acid is a by-product of the reaction of a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid. In order to provide an efficient economical process and a high molecular weight polyarylate, the acid, for example, the acetic acid has to be conveniently and efficiently removed.

It has now been discovered that a polyarylate having improved color and having a reduced viscosity of at least about 0.5 to greater than 1.0 dl/gm can be efficiently and economically produced. The present process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent, based on the weight of the polyarylate produced, of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain a benzylic and/or tertiary hydrogen atom, at a temperature of from about 260° to about 350° C. Optionally, the process may be carried out in the presence of a catalyst.

The utilization of between about 10 to about 60 weight percent of at least one of the cycloaliphatic, substituted aromatic, or heteroaromatic compounds and optionally, a catalyst, as will hereinafter be described, in the diacetate process, results in the production of a polyarylate which has improved color, and thus, the polyarylate can be used to produce molded articles of improved clarity. Also, the utilization of said compounds, in the diacetate process, prevents sublimation of the aromatic dicarboxylic acid; thus producing polyarylates of acceptable molecular weight and also provides for better removal of the acetic acid by-product. Further, an additional benefit in using the alkyl substituted aromatic, heteroaromatic or cycloaliphatic compounds in the amounts indicated is that the viscosity of the system is decreased. This decrease in viscosity provides a faster reaction time since better mixing of the reactants occurs which allows the reaction to proceed under kinetic control. Additionally, by using these compounds, reaction times are relatively short so that a polyarylate is produced generally in less than 10 hours at the reaction temperatures. Furthermore, the present process can be carried out at atmospheric pressure or supra atmospheric pressures and therefore avoids the use of the costly equipment which is needed by the prior art processes which carry out the diacetate processes under vacuum.

DESCRIPTION OF THE INVENTION

The present invention is directed to a process for the preparation of polyarylates having a reduced viscosity of from about 0.5 to greater than 1.0 dl/gm, which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent, based on the weight of the polyarylate produced, of at least one cycloaliphatic, substituted, aromatic, or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen atom, at a temperature of from about 260° to about 350° C. Optionally, the process may be carried out in the presence of a catalyst.

The present process comprises reacting:

(a) at least one diester derivative of a dihydric phenol having the following formula: ##STR2## wherein R is independently selected from an alkyl radical having from 1 to about 6 carbon atoms, preferably methyl, cycloalkyl having from 4 to about 7 carbon atoms, y is independently selected from alkyl groups of 1 to 4 carbon atoms, chlorine or bromine, z independently has a value of from 0 to 4, inclusive, and R' is independently selected from a divalent saturated aliphatic hydrocarbon radical, particularly alkylene or alkylidene radicals having from 1 to 8 carbon atoms, especially C(CH₃)₂, cycloalkylene, or cycloalkylidene radicals having up to and including 9 carbon atoms, O, S, SO, SO₂, and CO, x is 0 or 1; and

(b) at least one aromatic dicarboxylic acid.

The diester derivative of the dihydric phenol is prepared by reacting a dihydric phenol with an acid anhydride derived from acids containing from 2 to 8 carbon atoms under conventional esterification conditions. The preferred acid anhydride is acetic anhydride. Generally, the dihydric phenol is reacted with the acid anhydride in the presence of an esterification catalyst, either in the presence or absence of a solvent.

The dihydric phenols that may be used in this invention include the following:

2,2-bis-(4-hydroxyphenyl)propane,

bis-(2-hydroxyphenyl)methane,

bis-(4-hydroxyphenyl)methane,

bis-(4-hydroxy-2,6-dimethyl-3-methoxyphenyl)methane,

1,1-bis-(4-hydroxyphenyl)ethane,

1,2-bis-(4-hydroxyphenyl)ethane,

1,1-bis-(4-hydroxy-2-chlorophenyl)ethane,

1,1-bis-(3-methyl-4-hydroxyphenyl)ethane,

1,3-bis-(3-methyl-4-hydroxyphenyl)propane,

2,2-bis-(3-phenyl-4-hydroxyphenyl)propane,

2,2-bis-(3-isopropyl-4-hydroxyphenyl)propane,

2,2-bis-(2-isopropyl-4-hydroxyphenyl)propane,

2,2-bis-(4-hydroxyphenyl)pentane,

3,3-bis-(4-hydroxyphenyl)pentane,

2,2-bis-(4-hydroxyphenyl)heptane,

1,2-bis-(4-hydroxyphenyl)-1,2-bis-(phenyl)-propane,

4,4'-(dihydroxyphenyl)ether,

4,4'-(dihydroxyphenyl)sulfide,

4,4'-(dihydroxyphenyl)sulfone,

4,4'-(dihydroxyphenyl)sulfoxide,

4,4'-(dihydroxybenzophenone),

hydroquinone, and

naphthalene diols.

These dihydric phenols may be used individually or in any combination which when reacted with an aromatic dicarboxylic acid produces polyarylate polymers or copolymers that essentially do not crystallize during the reaction and recovery steps. The preferred dihydric phenol is 2,2-bis(4-hydroxyphenyl)propane.

The aromatic dicarboxylic acids that may be used in this invention include terephthalic acid, isophthalic acid, any of the naphthalene dicarboxylic acids and mixtures thereof, as well as alkyl substituted homologs of these carboxylic acids, wherein the alkyl group contains from 1 to about 4 carbon atoms, and acids containing other inert substituents such as halides, alkyl or aryl ethers, and the like. Preferably, mixtures of isophthalic and terephthalic acids are used. The isophthalic acid to terephthalic acid ratio in the mixture is about 20:80 to about 100:0, while the most preferred acid ratio is about 25:75 to about 50:50. Also, from about 0.5 to about 20 percent of aliphatic diacids containing from 2 to about 10 carbon atoms, such as adipic acid, sebacic acid, and the like may be additionally used in the polymerization reaction.

The dihydric phenols and aromatic dicarboxylic acids are selected so that the polyarylate produced remains in an essentially amorphous state during the polymerization reaction and recovery step.

The reaction of the diester derivative of a dihydric phenol with the aromatic dicarboxylic acid is carried out in the presence of from about 10 to about 60, preferably from about 25 to about 40, and most preferably, from about 30 to about 40 weight percent, based on the weight of the polyarylate produced, of at least one cycloaliphatic, substituted aromatic, or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen atom. These compounds have a boiling point of about 150° to about 350° C., preferably from about 180° to about 280° C., and most preferably from about 180° to about 220° C., at atmospheric pressure. Additionally, these compounds have a solubility parameter, at a temperature of 250° C., of ±4 within the solubility parameter of the polyarylate being produced. Solubility parameter is a measure for correlating polymer solvent interaction. It is defined in "Properties of Polymers," D. W. Van Krevelen, Elsevier Scientific Publishing Co., Amsterdam-Oxford-New York, 1976, pp. 141-155, as the square root of the cohesive energy density.

The cycloaliphatic compounds are of the following formulae: ##STR3## wherein R₁ is independently alkyl of 1 to 6 carbon atoms, cycloalkyl of 6 to 18 carbon atoms, and a is an integer of 1 or 2.

The substituted aromatic compounds are of the following formulae: ##STR4## wherein R₂ is independently alkyl of 1 to 6 carbon atoms, cycloalkyl of 6 to 18 carbon atoms and aralkyl or alkaryl of 7 to 18 carbon atoms and wherein the carbon atom of R₂ attached directly to the benzene nucleus has 1 or 2 attached hydrogen atoms, and b is an integer of 1 to 6. ##STR5## wherein R₃ is independently alkyl of 1 to 6 carbon atoms, cycloalkyl of 6 to 18 carbon atoms, c is an integer of 1 or 2, and n is an integer of 1 to 6

The heteroaromatic compounds are of the following formula: ##STR6## wherein A is S, O, or --CH═N--, R₄ is independently alkyl of 1 to 6 carbon atoms, or cycloalkyl of 6 to 18 carbon atoms and wherein the carbon atom of R₄ attached directly to the heteroaromatic nucleus has 1 or 2 attached hydrogen atoms, and d is an integer of 1 to 4.

The preferred compounds encompassed by structures (I) through (V) include xylenes, cumene, diethylbenzene, diisopropyl benzene, tetrahydronaphthalene or decahydronaphthalene.

Additionally, the cycloaliphatic, substituted aromatic or heteroaromatic compounds may be used with up to 90 weight percent of other compounds such as diphenyl ether, dimethylsulfone, etc.

The amount of said compounds could vary during the polymerization reaction. For example, it may be advantageous to increase progressively the amount of these solvents to maintain the reaction medium at constant viscosity.

The catalyst which may optionally be used in this invention is selected from magnesium, manganese, or zinc salts. The preferred catalyst is a magnesium salt of an inorganic or organic acid, or mixtures thereof. The salts of the organic acids include acetates, propionates, benzoates, oxalates, acetylacetonates, or mixtures thereof. The most preferred catalyst is magnesium acetate. The catalyst is present in the reaction in a catalytically effective amount which can be, for example, from about 1 to about 1000, preferably from about 10 to about 50, parts per million, based on the weight of the polyarylate produced.

The reaction of the diester derivative of the dihydric phenol with the aromatic dicarboxylic acid is performed with these reactants present in amounts of from about 0.85:1.00 to about 1.10:1:00, preferably from about 0.98:1.02 to about 1.20:0.98, diester derivative: aromatic dicarboxylic acid.

The process of this invention is carried out at a temperature of from about 260° to about 350° C. and preferably, from about 275° to about 295° C. The present process is generally conducted in an inert atmosphere (such as argon or nitrogen). The process is preferably carried out at a pressure sufficient to have the solvent refluxing at the reaction temperature. This pressure is generally from about 1 to about 150 psig.

The polymerization reaction is conducted for a period of time sufficient to produce a polyarylate having a reduced viscosity of at least about 0.5 to greater than 1.0 dl/gm, which time is generally less than about 10 hours. The reaction time is generally in the range of from about 4 hours to about 8 hours, depending on the particular polyarylate being prepared.

The polymerization reaction of this invention may be carried out batchwise or continuously and by using any apparatus desired. Moreover, the reactants may be added to the polymerization zone in any way or order desired as long as the polymerization takes place in the presence of from about 10 to about 60 weight percent of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain a benzylic and/or tertiary hydrogen atom, and optionally, a catalyst.

The diester derivative of the dihydric phenol may be formed, in situ, by adding the dihydric phenol together with the acid anhydride, an aromatic dicarboxylic acid, and at least one cycloaliphatic, substituted aromatic or heteroaromatic compound and, optionally, a catalyst to the reactor, and the reaction carried out in a single reaction zone under combined esterification and polymerization conditions as described above. Additionally, the diester derivative of the dihydric phenol may be first prepared and then an aromatic dicarboxylic acid and the cycloaliphatic, substituted aromatic, or heteroaromatic compound added directly to the same reaction vessel with the polymerization being carried out under the conditions described above.

The polyarylate polymer having a reduced viscosity of at least about 0.5 is recovered in its final form by methods well known to those skilled in the art, such as by direct devolatilization in an extruder (under conditions sufficient to remove the cycloaliphatic, substituted aromatic, or heteroaromatic compound), coagulation, spray drying, and the like.

The polyarylates may also be prepared by first forming a polyarylate prepolymer having a reduced viscosity of from about 0.1 to about 0.4 dl/gm, by reacting the diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of the cycloaliphatic, substituted aromatic, or heteroaromatic compound under the conditions described previously, for a reaction time of about 3 hours. The polyarylate prepolymer is then heated at temperatures of from about 300° to about 350° C. to obtain a polyarylate having a reduced viscosity of from about 0.5 to greater than 1.0 dl/gm.

Alternatively, the polyarylate prepolymer may be added directly, after its formation, to a vented extruder wherein the molecular weight is increased to form a polymer having a reduced viscosity of from about 0.5 to greater than about 1.0 dl/gm. For example, the molecular weight of polyarylate prepolymer is increased in the extruder at a temperature of from about 320° to about 350° C., under a vacuum of about 0.3 to about 2 mm Hg and a residence time of from about 10 to about 30 minutes.

The process of this invention produces polyarylates having a reduced viscosity of from about 0.5 to greater than about 1.0, preferably from about 0.6 to about 0.8 dl/gm, as measured in chloroform (0.5 g/dl chloroform) or other suitable solvent at 25° C. In those instances where the polyarylate is not soluble in chloroform, other solvents known in the art, such as parachlorophenol, phenol/tetrachlorethane (60:40), and the like, may be used. Reduced viscosities or the polyarylates measured in these solvents generally have the same range as those measured in chloroform.

The polyarylates may be prepared in the presence of materials such as molecular weight regulators, antioxidants, and the like.

The polyarylates obtained by the process of this invention may be used together with the well-known additives such as plasticizers, pigments, lubricating agents, mold release agents, stabilizers, inorganic fillers, and the like. These polyarylates may also be blended with other polymers.

EXAMPLES

The following examples serve to give specific illustrations of the practice of this invention but they are not intended in any way to limit the scope of this invention. In these examples, all parts and percentages are on a weight basis unless otherwise specificed.

COMPARATIVE EXAMPLE A Preparation of Prepolymer

The apparatus used herein consisted of a 500 ml round bottom flask containing a heater, a nitrogen inlet, mechanical stirrer, a vacuum jacketed vigreux column connected to a distillation head and collector flask. 124.8 grams of bisphenol-A diacetate, 33.2 grams of isophthalic acid, 33.2 grams of terephthalic acid, and 63 grams of diphenyl ether were added to the flask. The reaction mixture was purged with nitrogen for about 20 minutes. The reaction mixture was then heated to about 275° C. Acetic acid started to distill when the temperature reached about 260° C. The reactants were maintained at 275° C. for 2 hours. The temperature was then raised to about 290°-295° C. and the reactants maintained at this temperature for 2 hours. No apparent distillation of acetic acid was observed during the last hour. A thick yellow product formed. This was allowed to cool to ambient temperature.

Approximately 3 grams of the product was dissolved in methylene chloride (approximately 10 percent solids) and coagulated in isopropanol. The resultant material which was in the form of a fluff was washed with isopropanol two times and then dried at 120° C. (for 24 hours at <1 mm Hg pressure). The reduced viscosity of this polymer was measured in chloroform (0.50 gm/100 ml) at 25° C. and found to be 0.25 dl/gm.

Preparation of Polymer

To a test tube (25 by 200 mm.) fitted with a vacuum adapter, 6 grams of the prepolymer as prepared above was added. The pressure in the test tube was reduced to <0.4 mm Hg. After two minutes at ambient temperature, the test tube was heated to 345°-350° C. and the prepolymer was maintained at this temperature for 30 minutes. The test tube was allowed to cool to ambient temperature. A polymer was recovered by breaking the test tube. The polymer had a deep yellow color. The reduced viscosity of this polymer was meausred in chloroform (0.50 gm/dl) at 25° C. and found to be 0.70 dl/gm.

The color number of the polyarylate polymer was determined as follows: the polyarylate polymer was dissolved in methylene chloride, coagulated, washed in isopropanol and then dried in a vacuum oven for about 24 hours at 120° C. and <1 mm Hg. The polymer was dissolved in chloroform to give a 10 weight percent solution. The absorbance (transmittance) of the solutions was measured in a Fischer Scientific Electrophotometer Model No. 81. The absorbance was converted to a color number using a Table determined as follows: Samples of poly(ether sulfone) with known color numbers, as determined by ASTM D-1003, were dissolved in chlorobenzene to give 5, 10 and 15 weight percent solutions. The absorbance (transmittance) of the solution was measured in the Fischer Electrophotometer. The absorbance was plotted versus color number which give a straight line for each concentration. (The polyarylate polymers were dissolved in chloroform because these polymers are not completely soluble in chlorobenzene).

The color number of the polyarylate produced in this Control A was 200.

The Table shows the weight percent of solvent used, based on the polymer produced, the reduced viscosities of the prepolymer and polymer formed and the color number.

EXAMPLE 1

The procedure of Control A for forming a prepolymer and polymer was exactly repeated except that 54.0 g (30 weight percent) of a mixture of diphenyl ether and diisopropylbenzene (20:1) was used in place of diphenyl ether. The color number was determined by the procedure described in Control A.

The Table shows the weight percent of solvent used, based on the polymer produced, the reduced viscosities of the prepolymer and polymer formed and the color number.

CONTROL B

The procedure of Control A for forming a prepolymer was exactly repeated except that additionally, 100 parts per million, based on polyarylate produced, of magnesium acetate catalyst was charged to the reactor. The procedure of Control A for forming a polymer was exactly repeated.

The Table shows the weight percent of solvent used, based on the polymer produced, the reduced viscosities of the prepolymer and polymer formed and the color number.

CONTROL C

The procedure of Control A for forming a prepolymer was exactly repeated except that dimethyl sulfone was used instead of diphenyl ether and additionally, 100 parts per million, based on polyarylate produced, of magnesium acetate catalyst was charged to the reactor. The procedure of Control A for forming a polymer was exactly repeated.

The Table shows the weight percent of solvent used, based on the polymer produced, the reduced viscosities of the prepolymer and polymer formed and the color number.

EXAMPLES 2 to 5

The procedure as described in Control A for forming a prepolymer and polymer was exactly repeated in Examples 2 to 5 except that in forming the prepolymer, 100 parts per million, based on polyarylate produced, of magnesium acetate catalyst was charged to the reactor as well as the solvent and amounts thereof shown in the Table.

                                      TABLE                                        __________________________________________________________________________     Solvent                                                                                        Amount Catalyst.sup.1    Reduced Viscosity                                     Used   (100 parts                                                                            Reduced Viscosity                                                                         of Polymer                                                                               Color                       Example                                                                              Type .    (gm)                                                                              (wt. %)                                                                            per million)                                                                          of Prepolymer(dl/gm)                                                                      (dl/gm)   Number                      __________________________________________________________________________     Control A                                                                            diphenyl ether                                                                              35  No     0.25       0.70      200                         1     diphenyl ether/                                                                             30  No     0.30       0.69      50                                diisopropyl                                                                    benzene (20:1)                                                           Control B                                                                            diphenyl ether                                                                              35  Mg(OAc).sub.2                                                                         0.24       0.73      205                         Control C                                                                            dimethyl sulfone                                                                            35  Mg(OAc).sub.2                                                                         0.30       0.72      180                         2     diisopropyl  30  Mg(OAc).sub.2                                                                         0.31       0.67      48                                benzene                                                                  3     tetrahydro-  30  Mg(OAc).sub.2                                                                         0.34       0.66      86                          4     decahydro-   30  Mg(OAc).sub.2                                                                         0.35       0.61      130                               naphthalene                                                              5     dimethyl sulfone/                                                                           30  Mg(OAc).sub.2                                                                         0.29       0.67      65                                tetrahydronaphtha-                                                             lene (9:1)                                                               __________________________________________________________________________      .sup.1 Mg(OAc).sub.2 = magnesium acetate                                  

What is claimed is:
 1. A process for preparing polyarylate having a reduced viscosity of from about 0.5 to greater than 1.0 dl/gm and improved color which process comprises reacting, at a temperature of from about 260° to about 350° C.:(a) at least one diester derivative of a dihydric phenol having the following formula: ##STR7## wherein R is independently selected from an alkyl radical having from 1 to about 6 carbon atoms or cycloalkyl having from 4 to about 7 carbon atoms, y is independently selected from alkyl groups of 1 to 4 carbon atoms, chlorine, or bromine, z independently has a value of from 0 to 4, inclusive, and R' is independently selected from a divalent saturated aliphatic hydrocarbon radical having 1 to 8 carbon atoms, a cycloalkylene, or cycloalkylidene radical having up to and including 9 carbon atoms, O, S, SO, SO₂, CO, x is 0 or 1; with (b) at least one aromatic dicarboxylic acid, in the presence of from about 10 to about 60 weight percent, based on the polyarylate produced, of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen atom, and have a boiling point of about 150° to about 350° C. at atmospheric pressure.
 2. A process as defined in claim 1 wherein the diester derivative of a dihydric phenol has the following formula: ##STR8##
 3. A process as defined in claim 2 wherein each z is
 0. 4. A process as defined in claim 1 wherein the aromatic dicarboxylic acid is a mixture of isophthalic acid and terephthalic acid.
 5. A process as defined in claim 4 wherein the isophthalic acid to terephthalic acid ratio in the mixture is about 25:75 to about 100:0.
 6. A process as defined in claim 1 wherein the temperature is between about 275° to about 295° C.
 7. A process as defined in claim 1 wherein the cycloaliphatic compound is of the following formulae: ##STR9## wherein R₁ is independently alkyl of 1 to 6 carbon atoms, cycloalkyl of 6 to 18 carbon atoms, and a is an integer of 1 or
 2. 8. A process as defined in claim 1 wherein the substituted aromatic compound is of the following formulae: ##STR10## wherein R₂ is independently alkyl of 1 to 6 carbon atoms, cycloalkyl of 6 to 18 carbon atoms and aralkyl or alkaryl of 7 to 18 carbon atoms and wherein the carbon atom of R₂ attached directly to the benzene nucleus has 1 or 2 attached hydrogen atoms, and b is an integer of 1 to 6; ##STR11## wherein R₃ is independently alkyl of 1 to 6 carbon atoms, cycloalkyl of 6 to 18 carbon atoms, c is an integer of 1 or 2, and n is an integer of 1 to
 6. 9. A process as defined in claim 8 wherein the substituted aromatic compound is selected from xylene, cumene, diethylbenzene, diisopropyl benzene, tetrahydronaphthalene, or decahydronaphthalene.
 10. A process as defined in claim 1 wherein the heteroaromatic compound is of the following formula: ##STR12## wherein A is S, O, or --CH═N--, R₄ is independently alkyl of 1 to 6 carbon atoms, or cycloalkyl of 6 to 18 carbon atoms wherein the carbon atom of R₄ attached directly to the heteroaromatic nucleus has 1 or 2 attached hydrogen atoms, and d is an integer of 1 or
 2. 11. A process as defined in claim 1 wherein the polyarylate has a reduced viscosity of from about 0.6 to about 0.8.
 12. A process as defined in claim 1 wherein the cycloaliphatic, substituted aromatic or heteroaromatic compound is present in an amount of from about 25 to about 40 weight percent.
 13. A process for preparing a polyarylate having a reduced viscosity of from about 0.5 to greater than 1.0 dl/gm which process comprises reacting at a temperature of from about 260° to about 350° C.:(a) an acid anhydride based on an acid containing from 2 to 8 carbon atoms; (b) at least one dihydric phenol having the following formula: ##STR13## wherein y is independently selected from alkyl groups of 1 to 4 carbon atoms, chlorine or bromine, z independently has a value of from 0 to 4, inclusive, R' is independently selected from a divalent saturated hydrocarbon radical having 1 to 8 carbon atoms, a cycloalkylene or cycloalkylidene radical having up to and including 9 carbon atoms, O, S, SO, SO₂, Co, x is 0 or 1; and (c) at least one aromatic dicarboxylic acid, in the presence of from about 10 to about 60 weight percent, based on the polyarylate produced, of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen, and have a boiling point of about 150° to about 350° C. at atmospheric pressure.
 14. A process for preparing a polyarylate having a reduced viscosity of from about 0.5 to greater than 1.0 dl/gm, which process comprises the following steps:I. forming a polyarylate prepolymer having a reduced viscosity of from about 0.1 to about 0.4 dl/gm, by reacting:(a) at least one diester derivative of a dihydric phenol having the following formula: ##STR14## wherein R is independently an alkyl radical having from 1 to about 6 carbon atoms, or cycloalkyl having from 4 to about 7 carbon atoms, y is independently selected from alkyl groups of 1 to 4 carbon atoms, chlorine, or bromine, z independently has a value of from 0 to 4, inclusive, and R' is independently selected from a divalent saturated aliphatic hydrocarbon radical having 1 to 8 carbon atoms, a cycloalkylene, or cycloalkylidene radical having up to and including 9 carbon atoms, O, S, SO, SO₂, CO, x is 0 or 1; with (b) at least one aromatic dicarboxylic acid, in the presence of from about 10 to about 60 weight percent, based on the polyarylate produced, of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen, and have a boiling point of about 150° to about 350° C. at atmospheric pressure. II. Heating the prepolymer so formed at a temperature of from about 300° to 350° C. for a period of time sufficient to form a polyarylate having a reduced viscosity of from about 0.5 to greater than 1.0 dl/g.
 15. A process as defined in claim 14 wherein the prepolymer is heated in step (II) in a vented extruder under vacuum. 